Method for production of semiconductor package

ABSTRACT

The invention provides a method for producing semiconductor packages comprising the steps of forming electronic circuits for a plurality of semiconductor chips  11  on a wafer  1,  forming bumps  2  on the plurality of semiconductor chips  11,  encapsulating the circuit-forming surface  111  of the wafer  1  and the bumps  2  with a sealant by screen printing means to form a sealant layer  4,  curing the sealant layer  4,  grinding the surface of the sealant layer  4  until the upper end surface of the bump  2  becomes exposed, placing solder balls on said upper end surface of bumps  2  to weld the balls to the surface thereof, and dicing the wafer  1  and the sealant layer  4  as united into individual semiconductor chips  11.  Screen printing means is used to encapsulate the entire surface of the wafer with a resin, so that the equipment costs can be markedly reduced as compared with conventional methods using a mold.

FIELD OF THE INVENTION

The present invention relates to a method for producing a chip-sizesemiconductor package.

BACKGROUND ART

In view of recent development of electronic equipment such as portabletelephones, notebook-size personal computers, electronic personal databooks, etc., there are demands for production of semiconductor packagesof higher density, smaller size and reduced thickness which are usefulfor such electronic equipment.

To meet said demands, various kinds of semiconductor packages have beendeveloped. Examples are LSI-mounting TAB, tape carriers, plastic leadedchip carriers (PLCC), ball grid arrays (BGA), chip-size packages (CSP),flip chips, etc. These semiconductor packages have excellent featuresbut are defective in production efficiency and mounting reliability.

Methods have been proposed for producing chip-size semiconductorpackages among said semiconductor packages in an attempt to improve theproduction efficiency and the mounting reliability (of. JapaneseUnexamined Patent Publication No. 79362/1998). According to the proposedmethods, electronic circuits for a plurality of semiconductor chips areformed on a wafer and bumps are provided on the semiconductor chips.Then, after the wafer is placed into a mold cavity, a resin is suppliedto the space around the bumps to encapsulate the bumps by the so-calledtransfer molding. Thereafter at least the tips of bumps covered with theresin layer are exposed at the surface of the resin layer. Finally thewafer with the resin layer formed thereon is cut into individualsemiconductor chips to obtain semiconductor packages.

According to the foregoing conventional methods for producingsemiconductor packages, a resin layer is formed on the bump-arrangedsurface of the wafer before mounting the semiconductor packages on theprinted circuit board. Consequently the methods eliminate the need forthe step of filling an encapsulation resin into a narrow space betweenthe semiconductor chips and the printed circuit board after mounting thesemiconductor chips on the board so that the mounting reliability isimproved. Because of the encapsulation of the wafer with a resin, themethods can achieve a higher production efficiency than theencapsulation of individual semiconductor packages with a resin.

However, the conventional methods pose the following problems due to theuse of ai a mold. First of all, high investment in equipment isessentially needed. Further, since the encapsulating step andheat-curing step are conducted in this order within the mold, the waferis confined in the mold for a prolonged period of time, thereby loweringthe production efficiency. Moreover, it is difficult to form a resinlayer having a thickness of 1 mm or less.

DISCLOSURE OF THE INVENTION

The present invention was accomplished to overcome the above-mentionedprior art problems. An object of the invention is to provide a methodfor producing semiconductor packages, the method being capable oflowering the equipment investment, increasing the production efficiencyand forming a resin layer with a thickness of 1 mm or less.

Other features of the invention will become apparent from the followingdescription.

According to the invention, there is provided a method for producingsemiconductor package, the method comprising the steps of:

-   -   forming electronic circuits for a plurality of semiconductor        chips on a wafer;    -   forming bumps on the plurality of semiconductor chips,        encapsulating the circuit-forming surface of the wafer and bumps        with a sealant by screen printing means to form a sealant layer;    -   curing the sealant layer;    -   grinding the surface of the sealant layer until the upper end        surface of the bumps becomes exposed;    -   placing solder balls on said upper end surface of the bumps to        weld the balls to the surface thereof; and    -   dicing the wafer and the sealant layer as united into individual        semiconductor chips.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1 is a perspective view showing an example of semiconductorpackages produced by the method for producing semiconductor packagesaccording to the invention.

FIG. 2 is a flow chart showing an embodiment of the invention.

FIG. 3 is a perspective view schematically showing electronic circuitsas formed on the wafer in an embodiment of the method for producingsemiconductor packages according to the invention.

FIG. 4 is a longitudinal sectional view schematically showing the bumpsas formed on the circuit-forming side of the wafer.

FIG. 5 is a longitudinal sectional view schematically showing a sealantlayer as formed by forcedly filling a sealant using screen printingmeans.

FIG. 6 is a longitudinal sectional view schematically showing thesealant layer as cured on the wafer illustrated in FIG. 4.

FIG. 7 is a longitudinal sectional view schematically showing thesealant layer illustrated in FIG. 6 whose surface is being ground.

FIG. 8 is a longitudinal sectional view schematically showing solderballs as placed on and as welded to the upper end surface of bumpsillustrated in FIG. 7.

FIG. 9 is a longitudinal sectional view schematically showing the waferand the sealant layer united as being cut, one by one, into individualsemiconductor chips.

FIG. 10 is a perspective view schematically showing another example ofsemiconductor packages produced by the producing method of theinvention.

FIG. 11 is a longitudinal sectional view schematically showing a sealantlayer as being formed by feeding a sealant onto the wafer with bumps onthe circuit-forming surface of the wafer and forcedly spreading thesealant on the wafer by screen printing means in the course of producingthe semiconductor package as shown in FIG. 10.

FIG. 11a is an enlarged view schematically showing a portion surroundedwith a broken-line circle X in FIG. 11.

FIG. 11b is a longitudinal sectional view schematically showing thecross section shape of a portion of sealant layer existing at one end ofthe wafer after release of the metal mask.

FIG. 12 is a longitudinal sectional view schematically showing thesealant layer as cured on the wafer of FIG. 11.

FIG. 13 is a longitudinal sectional view schematically showing thesurface of the sealant layer of FIG. 12 as being ground.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention will be further clarified by the description of twoembodiments with reference to the accompanying drawings. The invention,however, is not limited to the embodiments, but various modificationsare possible without deviation from the scope of the invention.

FIG. 2 is a flow chart showing a first embodiment of the invention.FIGS. 3-9 schematically show respective steps of the method depicted inFIG. 2. FIG. 1 is a perspective view showing an example of semiconductorpackages produced by the producing method practiced stepwise asillustrated in FIG. 2.

First, the structure of a semiconductor package 10 is described belowwith reference to FIG. 1. The semiconductor package 10 comprises asemiconductor chip 11, bumps 2 serving as electrodes, a sealant layer 4having substantially the same height as the bumps 2 and solder balls 3welded to the upper end surface of bumps. Namely the semiconductorpackage 10 has a very simple structure.

The semiconductor package 10 with said structure is substantially equalin size to the semiconductor chip 11, and is of the so-called chip-sizepackage structure. Because of this structure, the semiconductor packageproduced by the method of the invention can satisfactorily fulfil theneed for the miniaturized semiconductor packages as required in recentyears.

The method for producing the semiconductor package 10 is described belowwith reference to FIGS. 2-9. The semiconductor package 10 is produced bycarrying out the steps shown in FIG. 2.

In the step of forming electronic circuits in FIG. 2, electroniccircuits (not shown) corresponding to a plurality of semiconductor chips11, e.g., a few hundreds of semiconductor chips, are formed on a wafer 1made of a silicone or the like as shown in FIG. 3. The electroniccircuits are formed by conventional techniques as by application ofexcimer laser.

After practicing the step of forming the electronic circuits, the wafer1 is subjected to the step of forming bumps. In the step of formingbumps, pillar-shaped bumps 2 with the specified height are provided onthe wafer surface 111 provided with the circuit as shown in FIG. 4. Thebumps 2 are formed by the bump-forming technique conventionally used forflip chips or the like, such as the plating method.

After practicing the step of forming bumps, the wafer 1 is subjected tothe step of printing encapsulation. In the printing encapsulation step,a viscous fluid sealant 40 is forcedly filled by screen printing to formon the entire circuit-forming surface of the wafer 1 a sealant layer 4having a thickness of height higher than the bumps 2 as shown in FIG. 5.Stated more specifically, after a metal mask 6 and the wafer 1 areproperly positioned, a specific amount of sealant 40 is fed onto thespecified part of the metal mask 6 and forcedly filled into athrough-hole 6a of the metal mask 6 by the reciprocative movement of asqueegee 5. After filling, the metal mask 6 is released from the sealantlayer 4. The diameter of the through-hole 6a is substantially equal toor smaller by about 1 to about 10 mm than that of the wafer. Preferredsealing materials useful as the sealant 40 are viscous fluids which areexcellent in adhesion to the wafer 1, and have low shrinkage in curing,low residual stress after curing, low expansion coefficient, low waterabsorption and high heat resistance. An epoxy resin composition havingsuch properties is suitable as the sealant 40. Specific examples includea composition containing an epoxy resin and silica in an amount of 60 to95% by weight based on the composition, such as NPR-780 and NPR-785(trademarks, products of Japan Rec Co., Ltd.). It may occur that the airis included into the sealant 40 when the sealant 40 is forcedly filledinto the through-hole 6a of the metal mask 6. The inclusion of air canbe effectively prevented by encapsulation with screen printing meansunder a vacuum atmosphere preferably in the same vacuum degree betweenduring the forward movement of the squeegee and during the backwardmovement thereof. Alternatively the vacuum degree may be varied betweenduring the forward movement of the squeegee and during the backwardmovement thereof (e.g. under 10 Torr or less in the forward movement andunder 50 to 150 Torr in the backward movement). In the method of theinvention, printing means is used for encapsulation, so that the sealantlayer can be thinned to a minimum thickness of about 50 μm. Whenrequired, a thick layer up to about 2 mm in thickness can be formed.

After practicing the step of printing encapsulation, the wafer 1 issubjected to the step of curing the sealant layer. In the step of curingthe sealant layer, the wafer 1 with the sealant layer is placed into aknown heating furnace to cure the sealant layer as shown in FIG. 6.

After practicing the step of curing the sealant layer, the wafer 1 issubjected to the step of exposing the bumps. In the step of exposing thebumps, the surface of the sealant layer 4 is ground with a grinder 7 asshown in FIG. 7 until the upper end surface of the bump 2 becomesexposed.

After practicing the step of exposing the bumps, the wafer 1 issubjected to the step of welding solder balls. In the step of weldingsolder balls, solder balls 3 for bonding to the printed circuit boardare placed onto the exposed upper end surface of the bumps and weldedthereto by the conventional ball mounter as shown in FIG. 8. The solderballs 3 can be welded to the bumps by the conventional bump-formingtechnique such as a transfer method.

Finally after executing the step of welding solder balls, the wafer 1 issubjected to the dicing step. In the dicing step, the wafer 1 and thesealant layer 4 as united are diced by a known dicer 8 into individualchips 11, whereby numerous chip-size semiconductor packages 10 areobtained.

A second embodiment of the invention will be described below. FIG. 10 isa perspective view schematically showing another example ofsemiconductor packages produced by the producing method of theinvention. A semiconductor package 100 shown in FIG. 10 is produced bypracticing the same steps of FIG. 2 as done in producing thesemiconductor package 10 of FIG. 1 except that the steps of printingencapsulation, curing the sealant layer and exposing the bumps aredifferent from the corresponding steps of the first embodiment. FIGS. 11to 13 are depicted for clarification of the different steps in thesecond embodiment.

First, the semiconductor package 100 according to the second embodimentis described with reference to FIG. 10. The semiconductor package 100comprises a semiconductor chip 11, bumps 2, a sealant layer 400 andsolder balls 3 which are provided in the semiconductor package 10 of thefirst embodiment. However, unlike the semiconductor package 10, theupper surface of the sealant layer 400 does not evenly extend and coversthe bumps 2 in such a manner that the bump 2 is individually surroundedwith a slope as shown in FIG. 10. With this structure, the semiconductorpackage 100 is such that the bumps 2 are reinforced by the sealant layer400 and the circuit-forming surface 111 of the chip 11 is covered andprotected with the sealant layer 400. Consequently the semiconductorpackage 100 is equal in mounting reliability to the semiconductorpackage 10 of the first embodiment shown in FIG. 1.

The method for producing the semiconductor package 100 is describedbelow. In the method for producing the semiconductor package 100, thesteps of forming electronic circuits and forming bumps as shown in FIG.2 are initially conducted, followed by the step of encapsulating thesurface of the wafer. The steps of forming electronic circuits andforming bumps are identical with those executed in producing thesemiconductor package 10 according to the first embodiment, Thus, thedescription of the steps is omitted. As shown in FIG. 11, a squeegee 5made of an elastic material such as rubber is vertically stretched orretracted to forcedly fill the sealant 40 into the through-hole 6a ofthe metal mask 6 in the printing encapsulation step of the secondembodiment. Then the sealant 40 is raked out to form the sealant layer400 which is concave and convex with the bumps 2 individually surroundedwith a slope. Stated more specifically, the squeegee 5 in the initialposition (FIG. 11, L₁) is retracted as strongly pressed against theupper surface of the metal mask 6. Then the squeegee 5 proceeds toforcedly fill the sealant 40 into the through-hole 6a and then stretchesdownward to rake out the sealant 40 so that each bump 2 is surroundedwith an inclined portion of the sealant layer 400. In the location voidof the bumps 2 (FIG. 11, L₂), the extended squeegee 5 partly scrape outthe sealant 40 so that the sealant layer 400 is concave and convex asshown in FIG. 10. On the bump 2 in position (FIG. 11, L₂), the squeegee5 is retracted against its top. After forming a thin layer of sealant 40atop the bumps 2, the squeegee 5 moves forward beyond the bump 2. Theend of the squeegee 5 has a shape with, for example, an inclined face asshown in FIG. 11 so that it can readily retract to prevent the bumps 2from carrying an excessive load. FIG. 11a is an enlarged viewschematically showing a portion surrounded with a broken-line circle Xin FIG. 11. For example, if the height H₁ of the bump 2 is 130 μm inFIG. 11a, the thickness H₂ of the sealant layer 400 on the upper surfaceof the bumps 2 may be about 20 μm and the thickness H₃ of the sealantlayer 400 in the location void of the bumps 2 may be about 50 μm. Toform a portion of sealant layer 400 at one end of the wafer 1 insubstantially the same cross section shape as that of sealant layer 400between the bumps 2, the metal mask 6 and the wafer 1 are adjusted invertical position so that the upper surface of the metal mask 6 is at alower level than the top of the bumps 2. FIG. 11b is a longitudinalsectional view schematically showing the cross section shape of aportion of sealant layer 400 at one end of the wafer 1 after release ofthe metal mask 6. When the upper surface of the metal mask 6 is broughtto a lower level than the top of the bumps 2, a sealant layer portion400a at one end of the metal mask 6 levels off to become substantiallyidentical in thickness with a sealant layer portion 400b between thebumps 2. In this way, it becomes possible to cut the wafer 1 with theresin layer into semiconductor packages 100 individually havingsubstantially the same cross section shape. When the sealant layer 400is formed according to the second embodiment, the sealant 40 will beconsumed in a reduced amount, whereby lower costs are involved inproducing semiconductor packages, as compared with the costs forproducing semiconductor packages according to the first embodiment. Thesecond embodiment can employ favorable techniques used in the firstembodiment such as materials of the sealant 40, and printingencapsulation under a vacuum atmosphere.

After practicing the step of printing encapsulation, the wafer 1 issubject(ed to the step of curing the sealant layer. In the step ofcuring the sealant layer, the printed wafer 1 is placed into a heatingfurnace to cure the sealant layer indented between the pairs of bumps asshown in FIG. 12.

After practicing the step of curing the sealant layer, the wafer 1 issubjected to the step of exposing the bumps. In the step of exposing thebumps, the surface of the sealant layer 400 is ground with the grinder 7as shown in FIG. 13 until the upper end surface of the bump 2 becomesexposed. After the layer 400 on the bumps 2 is ground to expose thebumps 2, preferably the top of bumps 2 is slightly ground to becomehorizontal, thereby facilitating welding of solder balls to the top ofbumps 2. Unlike the first embodiment, the portion to be ground islimited in the second embodiment to the layer on the upper surface ofbumps 2 and possibly to the surface of the bumps 2, whereby grinding isfacilitated. Further, a lower resistance to grinding is entailed,whereby the load on the bumps 2 is decreased and the possibility ofdamaging the bumps 2 is lowered.

After practicing the step of exposing the bumps, a plurality ofsemiconductor packages 100 are obtained following the step of weldingsolder balls and the dicing steps. The step of welding solder balls andthe dicing steps can be carried out in the same manner as in the firstembodiment. Thus the description of these steps is omitted.

According to the producing method of the invention, screen printingmeans is used to encapsulate the entire surface of the wafer with aresin, so that the equipment costs can be markedly reduced as comparedwith conventional methods using a mold. Since the formation of sealantlayer and the heat-curing are separately done, the production operationcan be continuously performed without necessity of confining the waferto a step for a prolonged period of time. Moreover, the sealant layercan be thinned to a minimum thickness of about 50 μm.

1. A method for producing semiconductor packages, comprising the stepsof : forming electronic circuits for a plurality of semiconductor chipson a surface of a semiconductor wafer; forming substantially columnarshaped bumps on circuit-provided the surface of the semiconductor waferin accordance with the electronic circuits by a plating method, eachbump having an upper end and a height; screen-printing the bump-providedsurface of the semiconductor wafer with a resin sealant to encapsulatethe bumps, including their the upper ends of the bumps, and theremaining circuit-provided electronic circuits formed on the surface ofthe semiconductor wafer with the resin sealant, by placing a metal maskhaving a single through-hole and a thickness which is larger than theheight of each bump over the surface of the semiconductor wafer, therebyforming a sealant layer; curing the sealant layer; grinding the curedsealant layer downward until the upper ends of the bumps become exposed;and placing solder balls on the exposed upper ends of the bumps to weldthe balls thereto; and dicing the semiconductor wafer and the sealantlayer as an integrated unit into individual semiconductor chips.
 2. Themethod according to claim 1, wherein the screen-printing step comprisesthe steps of : placing a mask having through-holes on the bump-providedsurface of the wafer; feeding the resin sealant onto the placed mask;and squeegeeing the resin sealant onto the bump-provided surface of thesemiconductor wafer through the through-holes single through-hole of themask by sliding a squeegee along the mask with a reciprocative movement.3. The method according to claim 2, wherein the squeegee is made ofelastic material and is passed over the bumps to make in a manner so asto cause the resin sealant to cave in between the bumps while being slidalong the mask.
 4. The method according to claim 1, wherein the resinsealant is a viscous fluid.
 5. The method according to claim 1, furthercomprising welding solder balls to the bumps.
 6. The method according toclaim 5, wherein the welding comprises placing the solder balls on theexposed upper ends of the bumps.
 7. The method according to claim 1,wherein the screen-printing is performed under a vacuum atmosphere. 8.The method according to claim 1, wherein the bumps have a substantiallysame width at a lower end as at the upper end, and a same specifiedheight.
 9. The method according to claim 1, wherein the singlethrough-hole has a size which is not larger than the size of the wafer.10. The method according to claim 9, wherein the single through-hole hasa diameter which is smaller by about 1 mm to about 10 mm than a diameterof the semiconductor wafer.
 11. The method according to claim 1, whereinthe resin sealant includes silica.
 12. The method according to claim 1,wherein the sealant layer is thinned to a minimum thickness of about 50μm.
 13. The method according to claim 1, wherein the cured sealant layeris ground to a thickness of no less than 50 μm.
 14. The method accordingto claim 1, wherein the resin sealant is screen printed on the entirecircuit-forming surface of the semiconductor wafer.
 15. The methodaccording to claim 1, wherein the semiconductor wafer comprises aperiphery, and the metal mask is placed on the periphery of thesemiconductor wafer.
 16. The method according to claim 1, wherein thesealant layer is formed on the entire circuit-forming surface of thewafer.
 17. The method according to claim 15, wherein the resin sealantincludes silica.
 18. The method according to claim 17, wherein theforming of the sealant layer comprises squeegeeing the resin sealantonto the surface of the semiconductor wafer through the singlethrough-hole of the mask by sliding a squeegee along the mask with areciprocative movement.
 19. A method comprising: forming electroniccircuits for a plurality of semiconductor chips on a surface of asemiconductor wafer; forming substantially columnar shaped bumps on thesurface of the semiconductor wafer by a plating method, each bump havingan upper end and a height; screen-printing the surface of thesemiconductor wafer with a resin sealant including silica in an amountof 60-95 % of the resin sealant by weight to encapsulate the bumps,including the upper ends of the bumps, and the electronic circuitsformed on the surface of the semiconductor wafer with the resin sealant,thereby forming a sealant layer; curing the sealant layer; grinding thecured sealant layer downward until the upper ends of the bumps becomeexposed; and dicing the semiconductor wafer and the sealant layer as anintegrated unit into individual semiconductor chips.
 20. The methodaccording to claim 19, wherein the screen-printing comprises squeegeeingthe resin sealant onto the surface of the semiconductor wafer by slidinga squeegee along a metal mask having a single through-hole.